In the production of glass, if a raw material powder scatters at the time of introducing it into a melting furnace, there will be problems such that the homogeneity of the glass composition tends to deteriorate, and the raw material is wasted. In order to avoid such problems, a method of granulating the raw material powder and using it in the form of granules has been proposed.
The method for producing granules (the granulation method) is mainly a spray dry method as a wet system or a dry granulation method as a quasi-dry system. The spray dry method is a method wherein by means of e.g. a pulverizing and stirring apparatus such as a ball mill, water is supplied to a glass raw material mixture, followed by stirring while pulverizing the glass raw material mixture, to prepare a raw material slurry, and by means of a spraying means such as a spray drier, the raw material slurry is sprayed into a high temperature atmosphere for drying and solidifying, to obtain granules.
In the dry granulation method, the raw material is put into a granulation apparatus of e.g. a stirring and granulating system, and water in an amount of from a few % to a few tens % is added, followed by stirring, whereby the raw material particles are agglomerated to form granule particles, which are aggregated to form granules. After taking out the obtained granules, moisture is removed, as the case requires, and further, screening is carried out to remove coarse particles, as the case requires.
The dry granulation method has no raw material pulverizing step and thus is characterized in that particles used as the raw material would become granules as their size is maintained, and therefore, it is preferentially used to form granule particles having a particle size relatively larger than by the spray dry method. Further as compared with the spray dry method, it is advantageous in that the energy required for drying may be less, and the size of the entire installation is relatively compact so that the granulation installation may be installed in a factory, whereby the costs will be relatively low as no transportation costs by truck, etc. are required.
The following Patent Document 1 discloses a method wherein in order to improve the degree of homogeneity of glass and to reduce defects, a silica raw material powder to be used for the dry granulation method is finely pulverized so that D10 of the obtainable granules is made to be from 5 to 30 times of D90 of the silica raw material powder.
The following Patent Document 2 relates to a method for producing glass by forming calcium magnesium silicate by a solid-state reaction at the time of heating granules of a glass raw material mixture, and discloses that in order to promote the thermodynamic superiority for the solid-state reaction to a standard glass reaction, D90 of a calcium source material to be used for the dry granulation method is made to be less than 75 μm.